The present invention pertains to a variable-gain amplifier in which the gain is changed in accordance with the input signal.
FIG. 5 is a schematic circuit diagram illustrating an example of a conventional variable-gain amplifier.
In FIG. 5, Q1-Q6 represent npn transistors; R1-R3 represent resistors; SC1 and SC2 represent constant current sinks; T1-T3 and T1xe2x80x2-T3xe2x80x2 represent terminals.
The base of npn transistor Q1 is connected to terminal T1, and its emitter is connected to reference potential GND via constant current sink SC1.
The base of npn transistor Q2 is connected to terminal T1xe2x80x2, and its emitter is connected to reference potential GND via constant current sink SC2.
Resistor R3 is connected between the emitters of npn transistor Q1 and npn transistor Q2.
The emitters of npn transistor Q3 and npn transistor Q4 are connected together and to the collector of npn transistor Q1. Also, the base of npn transistor Q3 and the base of npn transistor Q4 are connected to terminals T2 and T2xe2x80x2, respectively.
The emitters of npn transistor Q5 and npn transistor Q6 are connected together and to the collector of npn transistor Q2. Also, the base of npn terminal Q5 and the base of npn transistor Q6 are connected to terminals T2xe2x80x2 and T2, respectively.
The collector of npn transistor Q3 is connected to power supply Vcc via resistor R1 and to terminal T3.
The collector of npn transistor Q6 is connected to power supply Vcc via resistor R2 and to terminal T3xe2x80x2.
The collectors of npn transistor Q4 and npn transistor Q5 are directly connected to power supply Vcc.
When the variable-gain amplifier shown in FIG. 5 with the configuration is used, the differential voltage v1 input between terminals T1 and T1xe2x80x2 is amplified by the gain G1 and output as differential voltage v3 that appears between terminals T3 and T3xe2x80x2. The gain G1 can vary corresponding to the differential voltage v2 input between terminals T2 and T2xe2x80x2.
First, the relationship between gain G1 and differential voltage v2 will be explained.
When the differential amplifier comprised of the transistor pair of npn transistor Q3 and npn transistor Q4 operates outside the saturation region, current I1 flowing through resistor R1 can be approximated by the following equation.                                                                         I1                =                                  I3                  /                                      {                                          1                      +                                              exp                        ⁡                                                  (                                                                                                                    -                                v2                                                            /                              V                                                        ⁢                                                          xe2x80x83                                                        ⁢                            T                                                    )                                                                                      }                                                                                                                          =                                  I3                  xc3x97                                      A                    ⁡                                          (                      v2                      )                                                                                                          "AutoLeftMatch"                            (        1        )            
where
VT=kT/qxe2x80x83xe2x80x83(2)
A(v2)=1/{1+exp (xe2x88x92v2/VT)}xe2x80x83xe2x80x83(3)
VT in Equation (1) represents the thermal voltage of the transistor. It is expressed as shown in Equation (2) using Boltzmann""s constant k, the junction temperature T of the transistor, and the charge q on the electron.
Current I2 flowing through resistor R2 can be approximated using the following equation similar to Equation (1).
I2=I3xe2x80x2xc3x97A(v2)xe2x80x83xe2x80x83(4)
Also, when equal currents Isc flow in constant current sinks SC1 and SC2, the collector current I3 of npn transistor Q1 and the collector current I3xe2x80x2 of npn transistor Q2 can be approximated using the following equations.
I3=Isc+Iexe2x80x83xe2x80x83(5)
I3xe2x80x2=Iscxe2x88x92Iexe2x80x83xe2x80x83(6)
where Ie represents the current flowing through resistor R3.
Also, differential voltage v1 is expressed as follows using the base-emitter voltage Vbe1 of npn transistor Q1, base-emitter voltage Vbe2 of npn transistor Q2, and resistance r3 of resistor R3.
V1=Vbe1r3xc3x97Iexe2x88x92Vbe2xe2x80x83xe2x80x83(7)
When resistors R1 and R2 have the same resistance r1, the differential voltage v3 is expressed as follows.
V3=(I2xe2x88x92I1)xc3x97r1xe2x80x83xe2x80x83(8)
In this case, differential voltage v1 varies by as much as xcex94v1. If the voltage change of base-emitter voltage Vbe1 and base-emitter voltage Vbe2 is small compared with voltage xcex94v1, voltage xcex94v1 can be expressed as follows.
xcex94v1=r3xc3x97xcex94Iexe2x80x83xe2x80x83(9)
xcex94Ie represents the change in current Ie corresponding to the change in voltage xcex94v1.
Also, the change xcex94I3 in current I3 and the change xcex94I3xe2x80x2 in current I3xe2x80x2 are expressed by the following equations.                                                                                           Δ                  ⁢                                      xe2x80x83                                    ⁢                  I3                                =                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  I                  ⁢                                      xe2x80x83                                    ⁢                  e                                                                                                        =                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                                      v1                    /                    r3                                                                                      "AutoLeftMatch"                            (        10        )                                                                                    Δ                ⁢                                  xe2x80x83                                ⁢                                  I3                  xe2x80x2                                            =                              Δ                ⁢                                  xe2x80x83                                ⁢                I                ⁢                                  xe2x80x83                                ⁢                e                                                                                        =                                                -                  Δ                                ⁢                                  xe2x80x83                                ⁢                                  v1                  /                  r3                                                                                        (        11        )            
Based on Equations (10) and (11), the change xcex94I1 in current I1 and the change xcex94I2 in current I2 can be expressed as follows.                                                                                           Δ                  ⁢                                      xe2x80x83                                    ⁢                  I1                                =                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  I3                  xc3x97                                      A                    ⁡                                          (                      v2                      )                                                                                                                                              =                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  v1                  xc3x97                                                            A                      ⁡                                              (                        v2                        )                                                              /                    r3                                                                                      "AutoLeftMatch"                            (        12        )                                                                                                      Δ                  ⁢                                      xe2x80x83                                    ⁢                  I2                                =                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                                      I3                    xe2x80x2                                    xc3x97                                      A                    ⁡                                          (                      v2                      )                                                                                                                                              =                                                      -                    Δ                                    ⁢                                      xe2x80x83                                    ⁢                  v1                  xc3x97                                                            A                      ⁡                                              (                        v2                        )                                                              /                    r3                                                                                      "AutoLeftMatch"                            (        13        )            
Based on Equations (8), (12), and (13), the change xcex94v3 of differential voltage v3 can be expressed as follows.                                                                                           Δ                  ⁢                                      xe2x80x83                                    ⁢                  v3                                =                                                      (                                                                  Δ                        ⁢                                                  xe2x80x83                                                ⁢                        I2                                            -                                              Δ                        ⁢                                                  xe2x80x83                                                ⁢                        I1                                                              )                                    xc3x97                  r1                                                                                                        =                                                      -                    2                                    ⁢                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  v1                  xc3x97                                      A                    ⁡                                          (                      v2                      )                                                        xc3x97                                      r1                    /                    r3                                                                                                                          =                                  G1                  xc3x97                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  v1                                                                    "AutoLeftMatch"                            (        14        )            
where
G1=xe2x88x922A(v2)xc3x97r1/r3xe2x80x83xe2x80x83(15)
Consequently, as shown in Equation 15, the gain G1 of variable-gain amplifier 10 shown in FIG. 5 can be changed corresponding to differential voltage v2.
The voltage drop Vt3 at terminal T3 with respect to power supply Vcc and the voltage drop Vt3xe2x80x2 at terminal T3xe2x80x2 with respect to power supply Vcc are expressed as followed on the basis of the equation.                                                                         Vt3                =                                  r1                  xc3x97                  I1                                                                                                        =                                                      I                    ⁢                                          xe2x80x83                                        ⁢                    s                    ⁢                                          xe2x80x83                                        ⁢                    c                    xc3x97                    r1                    xc3x97                                          A                      ⁡                                              (                        v2                        )                                                                              +                                      I                    ⁢                                          xe2x80x83                                        ⁢                    e                    xc3x97                    r1                    xc3x97                                          A                      ⁡                                              (                        v2                        )                                                                                                                                "AutoLeftMatch"                            (        16        )                                                                                    Vt3                xe2x80x2                            =                              r1                xc3x97                I2                                                                                        =                                                I                  ⁢                                      xe2x80x83                                    ⁢                  s                  ⁢                                      xe2x80x83                                    ⁢                  c                  xc3x97                  r1                  xc3x97                                      A                    ⁡                                          (                      v2                      )                                                                      -                                  I                  ⁢                                      xe2x80x83                                    ⁢                  e                  xc3x97                  r1                  xc3x97                                      A                    ⁡                                          (                      v2                      )                                                                                                                              (        17        )            
In Equations (16) and (17), the second term represents a signal component that includes current Ie, which varies corresponding to the change in differential voltage v1, and the first term represents a certain in-phase component independent of the change in differential voltage v1. Also, the in-phase component varies corresponding to differential voltage v2. Consequently, when differential voltage v2 is changed in order to change the gain of the variable-gain amplifier, the in-phase voltage of the output also changes correspondingly. Since there is a limit on the range of the allowable in-phase voltage in the next stage of circuit that receives differential voltage v3, it is necessary to limit the dynamic range of differential voltage v3 or gain G1 to keep the in-phase voltage within that range.
The circuit shown in FIG. 6 is used to solve this problem of variable-gain amplifier 10 shown in FIG. 5.
The same symbols in FIGS. 5 and 6 represent the same respective elements. Also, in FIG. 6, Q7-Q10 represent npn transistors, and SC3 represents a constant current sink.
The emitters of npn transistors Q7-Q10 are connected to each other and to reference potential GND via constant current sink SC3.
Also, the bases of npn transistor Q7 and npn transistor Q8 are connected to terminal T2, and the collectors are connected to power supply Vcc.
The base of npn transistor Q9 is connected to terminal T2xe2x80x2, and the collector is connected to the collector of npn transistor Q3.
The base of npn transistor Q10 is connected to terminal T2xe2x80x2, and the collector is connected to the collector of npn transistor Q6.
In the variable-gain amplifier 11 shown in FIG. 6, when the same current Isc as that flowing in constant current sinks SC1 And SC2 flows in constant current sink SC3, the collector current I4 of npn transistor Q9 and the collector current I5 of npn transistor Q10 can be expressed as follows.
I4=I5=Iscxc3x97Axe2x80x2 (v2)xe2x80x83xe2x80x83(18)
where
Axe2x80x2(v2)=1/{1+exp (v2/VT)}xe2x80x83xe2x80x83(19)
Also, differential voltage v3 is expressed as follows.
V3=(I2+I5xe2x88x92I1xe2x88x92I4)xc3x97r1xe2x80x83xe2x80x83(20)
Since currents I4 and I5 are constant and independent of differential voltage v1, the change xcex94v3 in differential voltage v3 with respect to the change xcex94v1 in differential voltage v1 is the same as that expressed by Equation (14), and gain G1 will be the same as is expressed by Equation (15). Consequently, in the variable-gain amplifier 11 shown in FIG. 6, gain G1 can also be changed corresponding to differential voltage v2 in the same way as shown in FIG. 5.
Also, the voltage drop Vt3 at terminal T3 with respect to power supply Vcc and the voltage drop Vt3xe2x80x2 at terminal T3 with respect to power supply Vcc are expressed as follows.                                                                         Vt3                =                                  r1                  xc3x97                                      (                                          I1                      +                      I4                                        )                                                                                                                          =                                                      I                    ⁢                                          xe2x80x83                                        ⁢                    s                    ⁢                                          xe2x80x83                                        ⁢                    c                    xc3x97                    r1                    xc3x97                                          {                                                                        A                          ⁡                                                      (                            v2                            )                                                                          +                                                                              A                            xe2x80x2                                                    ⁡                                                      (                            v2                            )                                                                                              }                                                        +                                      I                    ⁢                                          xe2x80x83                                        ⁢                    e                    xc3x97                    r1                    xc3x97                                          A                      ⁡                                              (                        v2                        )                                                                                                                                                                    =                                                      I                    ⁢                                          xe2x80x83                                        ⁢                    s                    ⁢                                          xe2x80x83                                        ⁢                    c                    xc3x97                    r1                                    +                                      I                    ⁢                                          xe2x80x83                                        ⁢                    e                    xc3x97                    r1                    xc3x97                                          A                      ⁡                                              (                        v2                        )                                                                                                                                "AutoLeftMatch"                            (        21        )                                                                                    Vt3                xe2x80x2                            =                              r1                xc3x97                                  (                                      I2                    +                    I5                                    )                                                                                                        =                                                I                  ⁢                                      xe2x80x83                                    ⁢                  s                  ⁢                                      xe2x80x83                                    ⁢                  c                  xc3x97                  r1                  xc3x97                                      {                                                                  A                        ⁡                                                  (                          v2                          )                                                                    +                                                                        A                          xe2x80x2                                                ⁡                                                  (                          v2                          )                                                                                      }                                                  -                                  I                  ⁢                                      xe2x80x83                                    ⁢                  e                  xc3x97                  r1                  xc3x97                                      A                    ⁡                                          (                      v2                      )                                                                                                                                              =                                                I                  ⁢                                      xe2x80x83                                    ⁢                  s                  ⁢                                      xe2x80x83                                    ⁢                  c                  xc3x97                  r1                                -                                  I                  ⁢                                      xe2x80x83                                    ⁢                  e                  xc3x97                  r1                  xc3x97                                      A                    ⁡                                          (                      v2                      )                                                                                                                              (        22        )            
In Equations (21) and (22), the second term represents a signal component that varies corresponding to the change in differential voltage v1, and the first term represents a certain in-phase component that is independent of the change in differential voltage v1. The in-phase component, unlike Equations (16) and (17), also does not change with differential voltage v2. Consequently, the in-phase voltage of the output independent of gain G1 is constant. In this way, the problem of the variable-gain amplifier 10 shown in FIG. 5 is solved.
In the variable-gain amplifier 11 shown in FIG. 6, however, since not only the collector capacitance of npn transistor Q3 and npn transistor Q6 but also the collector capacitance of npn transistor Q9 and npn transistor Q10 are added to the output terminal of differential voltage v3, the frequency characteristic deteriorates compared with the variable-gain amplifier 10 shown in FIG. 5.
A general object of the present invention is to provide a variable-gain amplifier that can prevent the change in the in-phase voltage of the output occurring when the gain is changed and improve the frequency characteristic compared with the conventional method.
This and other objects and features are provided, in accordance with one aspect of the present invention by a variable-gain amplifier having the following: a differential current output circuit that outputs a first current and a second current as differential currents corresponding to a first input signal, a first current converter that converts the first current to a third current at a current conversion rate corresponding to a second input signal and outputs the third current to a first node, a second current converter that converts the second current to a fourth current at a current conversion rate corresponding to the second input signal and outputs the fourth current to a second node, a first load resistor connected between the first node and a third node, a second load resistor connected between the second and third nodes, and a voltage-adjustment circuit, which has an adjustable resistor between the voltage supply terminal and the third node and outputs a current corresponding to the second input signal to the third node such that the voltages at the first and second nodes do not change as the second signal varies.
In an aspect of the variable-gain amplifier of the present invention, the differential current of the first and second currents varies corresponding to the first input signal. As a result, the differential current of the third and fourth currents also changes. The voltage drops caused by the first and second load resistors change, and thus the differential voltage between the first and second nodes changes. The gain of the differential voltage with respect to the first input signal changes corresponding to the current conversion rates (current conversion gains) of the first and second current converters. When the current conversion gain changes corresponding to the second input signal, the current supplied to the third node is controlled corresponding to the second input signal such that the voltages at the first and second nodes become constant with respect to this change in the voltage-adjustment circuit. Consequently, when the gain of the differential voltage is changed with respect to the first input signal, the changes in the voltages at the first and second nodes are restricted. Also, since the voltage-adjustment circuit adjusts the voltages at the first and second nodes via the first and second load resistors, the deterioration of the frequency characteristic caused by the output capacitance of the voltage-adjustment circuit is alleviated.
In an aspect of the present invention the voltage-adjustment circuit may also have a pair of differential transistors to which the second input signal is supplied as a differential signal, and one of the transistors supplies a current corresponding to the second input signal to the third node.
In an aspect of the present invention, the first current converter may also have a pair of differential transistors to which the second input signal is supplied as a differential signal. One of the transistors supplies the third current to the first node. The second current converter may also have a pair of differential transistors to which the second input signal is supplied as a differential signal. One of the transistors supplies the fourth current to the second node.
In an aspect of the present invention, the differential current output circuit may also have a pair of differential transistors, to which the first input signal is supplied as a differential signal, and a resistor connected between the differential transistors. The differential transistors supply the first and second currents to the first and second current converters, respectively.